Thanks Wayne, so your idle is set purely by the set screw and not using the idle air bypass at all ?

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Richard that is correct. I find the idle and low speed operation is better w/o the air bypass screw. Simple adjustment by set screw under the butterfly...... again I use a meter and not just the tach...... my tach reads a tad high. My vin plate reads June 1981 although the papers always had 1982 since birth....

Lifetime Rossa

...so your idle is set purely by the set screw and not using the idle air bypass at all ?

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That's the procedure shown in the 281/83 Mondial8/QV WSM, but later procedures for models equipped with Bosch CIS systems (even others with K-Jet without Lambda) show setting a reduced RPM with the air bypass closed and then opening the air bypass to bring it up to the 1000 RPM. It's not like you couldn't try it both ways and see if there's any significant difference in behavior is it?

Lifetime Rossa

While we are in the topic of the bypass screw. If you can obtain idle without it... what was the designed function? Just asking....

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Couple of reasons that I can think of (but just speculating):

1. On systems with multiple throttle bodies that don't have a mechanical way to balance to two banks (i.e., a way to mechanically adjust the two throttle plates relative to each other), it gives a way to balance (equalize) the idle airflow of each bank. I can't recall anyone posting the 512BBi WSM (or I'd have grabbed it ), but that would be another interesting reference to check also being a K-Jet without Lambda system (so if someone's got it please post), and

2. In theory, having the throttle plate opened as little as possible at idle reduces the change in RPM if the mechanical position of the throttle plate varies at the idle position. Having some air flow thru the air bypass requires the throttle plate to be less opened (to keep the same 1000 RPM idle). This graph:Image Unavailable, Please Login
from this paper (page 14):

shows how the throttle open area changes versus throttle plate angle. The lower the throttle plate angle, the lower the slope of the curve, so any uncertainty in the throttle plate angle results in a smaller change of the open area.

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After reading another post I was prompted to look at the Mondial 8 WSM and found the following re idle speed. In 30 odd years of being a mechanic I can't recall another vehicle where the base idle was set by the throttle plate set screw with the idle air bypass not used Image Unavailable, Please LoginImage Unavailable, Please Login

Lifetime Rossa

I think you can conclude that the text of that Fig 11 was (wrongly) copied from another application using multiple throttle bodies as there is nothing to "balance" on a Mondial with a single throttle body and single intake plenum.

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Well while the motor was warm after a drive I set the throttle plate screw to 750 rpm and adjusted the air screw to 1050. I will have wait until dead cold to see how it interacts with cold starts.

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I set the base idle with the air, then blip the throttle and watch the fall-off RPMs to set the throttle. Basically, using the throttle screw to control the drop on decel, so that it doesn't go too far down. I don't want the throttle screw to SET the idle RPM; I want it to keep the throttle falloff from going so far that the engine dies.

Okay - dug up some notes from +10 yrs back, I believe it was from a dialogue with Fletch when I had my WUR & FD rebuilt. (Set the throttle plate so it just is not touching (.004”/.101mm) he mentioned a thin strip of paper would do. Just do not want the plate to seat. Then set the idle speed with the air idle screw.)

I removed and cleaned the air idle screw and replaced the rubber ring at the end of the spring for better sealing to the housing and the air screw threads.

I was able to set the idle at 1000 using a multi meter. Holds stable..... this works for me .... just saying...

Consultant

1. Close the bypass screw, set the idle screw to 500 RPM, open bypass screw to bring idle speed to 1000 RPM
2. Set idle screw to have a paper thin gap to the throttle lever, set idle speed to 1000 RPM using only the air bypass screw
3. Set idle speed using air bypass screw, and then supplement that with a bit of idle screw to help in deceleration

They are drastically different, and cannot all be correct. What gives?

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1. Close the bypass screw, set the idle screw to 500 RPM, open bypass screw to bring idle speed to 1000 RPM
2. Set idle screw to have a paper thin gap to the throttle lever, set idle speed to 1000 RPM using only the air bypass screw
3. Set idle speed using air bypass screw, and then supplement that with a bit of idle screw to help in deceleration

They are drastically different, and cannot all be correct. What gives?

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Typically in my experience
1. would suit non electronically controlled systems, usually used to to stop "running on"
2. would be used in a full electronic control system using an idle speed control motor. Throttle stop set to stop the plate binding in the body, but other than that engine idle speed controlled by ECU
3. Cant think of a car off the top of my head that uses that setup

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2. Set idle screw to have a paper thin gap to the throttle lever, set idle speed to 1000 RPM using only the air bypass screw
3. Set idle speed using air bypass screw, and then supplement that with a bit of idle screw to help in deceleration

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#2 was the first method I tried. The result was close, but not optimal. Therefore, #3 is a step that follows #2 on my car that resulted in my desired idle performance.

Consultant

The CIS system being one step above carburetors, it does not have any Idle Control Valve like DME electronics. That would lend itself to method 1) using both the idle screw and bypass screw to set the idle speed.